Work on the long-range document comes amid an unresolved legal challenge that threatens to unravel much of what SANDAG has formulated in recent years. The challenge centers on the agency’s 2050 Regional Transportation Plan, which was approved in 2011 and prioritizes the region’s freeway and mass-transit projects. That plan is a key template for San Diego Forward, which will serve as a legally required update to past versions.

Opponents of the 2050 Regional Transportation Plan said it pushes mass-transit projects to the back burner in favor of a vast expansion of the region’s freeways in the near term.

In December, a San Diego County Superior Court judge ruled that the transportation plan fails to comply with state environmental laws designed to reduce greenhouse-gas emissions.

SANDAG appealed the ruling, and no court date has been set for that process, said Jack Shu of the Cleveland National Forest Foundation, the advocacy group that brought the suit.

SANDAG leaders said their plan balances all types of transportation and allocates an unprecedented $106 billion for mass transit.

Colleen Windsor, a SANDAG spokeswoman, said Wednesday that work on court briefs was expected to last throughout 2013. She added that the agency is compelled to update its regional transportation plan every four years.

Along with transportation, “San Diego Forward” would address the region’s land use, public health, environment, housing, social equity, borders and military communities, according to a SANDAG news release.

“This new regional plan will build upon local planning efforts, and incorporate emerging issues and innovative concepts, to form an overall vision for the region’s future, including specific actions to turn that vision into reality,” the release said.